1. Field of the Invention
The present invention relates to a selective signal receiver for use in telecommunications and particularly in telephone systems. With the present invention, the time distances between zero crossings of the input voltage are measured with a counter which is controlled at a clock frequency higher than the signal frequency to be determined, and the input voltage is evaluated as a signal only if the measured value lies between a lower and an upper count determining the bandwidth.
2. Description of the Prior Art
Such signal receivers are disclosed in German Printed Applications Nos. 2,145,886 and 2,156,123, by way of example. In these known signal receivers, the counter determines during each measurement the time distance between two adjacent zero crossings or between the zero crossings at the beginning and at the end of a single cycle of the input voltage. With this prior art problems arise since, in addition to the signal frequencies, interfering signals frequently appear at the inputs of signal receivers. These interfering signals may shift the zero crossings of the input voltage to such an extent that the known digital signal receivers cannot make an accurate signal recognition.